Hanxuan Lin

Visualization of a ferromagnetic metallic edge state in manganite strips

Recently, broken symmetry effect induced edge states in two-dimensional electronic systems have attracted great attention. However, whether edge states may exist in strongly correlated oxides is not yet known. In this work, using perovskite manganites as prototype systems, we demonstrate that edge states do exist in strongly correlated oxides. Distinct appearance of ferromagnetic metallic phase is observed along the edge of manganite strips by magnetic force microscopy. The edge states have strong influence on the transport properties of the strips, leading to higher metal-insulator transition temperatures and lower resistivity in narrower strips. Model calculations show that the edge states are associated with the broken symmetry effect of the antiferromagnetic charge-ordered states in manganites. Besides providing a new understanding of the broken symmetry effect in complex oxides, our discoveries indicate that novel edge state physics may exist in strongly correlated oxides beyond the current two-dimensional electronic systems.

Manipulating electronic phase separation in strongly correlated oxides with an ordered array of antidots

Significance Electronic phase separation (EPS) is one of the most intriguing properties in complex materials. Great efforts have been made to understand or manipulate EPS, but how these phases are created and grow during percolation, let alone artificial control/fabrication of these phases, is still mysterious. In this work, we use a conceptual approach, i.e., fabricating antidots in manganites, and use their ferromagnetic metallic edge states to control the nucleation and growth of EPS domains. Consequently, we are able to tune the physical properties of the system without using external fields or changing doping concentration. The interesting transport and magnetic properties in manganites depend sensitively on the nucleation and growth of electronic phase-separated domains. By fabricating antidot arrays in La0.325Pr0.3Ca0.375MnO3 (LPCMO) epitaxial thin films, we create ordered arrays of micrometer-sized ferromagnetic metallic (FMM) rings in the LPCMO films that lead to dramatically increased metal–insulator transition temperatures and reduced resistances. The FMM rings emerge from the edges of the antidots where the lattice symmetry is broken. Based on our Monte Carlo simulation, these FMM rings assist the nucleation and growth of FMM phase domains increasing the metal–insulator transition with decreasing temperature or increasing magnetic field. This study points to a way in which electronic phase separation in manganites can be artificially controlled without changing chemical composition or applying external field.

Emerging single-phase state in small manganite nanodisks

Significance Electronic phase separation (EPS) is a common phenomenon in complex oxides systems. However, little is known regarding how EPS responds when the size of the system is smaller than the characteristic size of EPS. This issue is not only important for understanding the physical origin of EPS but also for oxides device applications in which oxides have to be fabricated into small-sized structures. Our work on manganites shows a surprising transition from the EPS state to a single phase state when the spatial size of the system is smaller than the characteristic length scale of EPS. This observation paves a way to manipulate EPS, which is potentially useful for oxides electronic and spintronic device applications. In complex oxides systems such as manganites, electronic phase separation (EPS), a consequence of strong electronic correlations, dictates the exotic electrical and magnetic properties of these materials. A fundamental yet unresolved issue is how EPS responds to spatial confinement; will EPS just scale with size of an object, or will the one of the phases be pinned? Understanding this behavior is critical for future oxides electronics and spintronics because scaling down of the system is unavoidable for these applications. In this work, we use La0.325Pr0.3Ca0.375MnO3 (LPCMO) single crystalline disks to study the effect of spatial confinement on EPS. The EPS state featuring coexistence of ferromagnetic metallic and charge order insulating phases appears to be the low-temperature ground state in bulk, thin films, and large disks, a previously unidentified ground state (i.e., a single ferromagnetic phase state emerges in smaller disks). The critical size is between 500 nm and 800 nm, which is similar to the characteristic length scale of EPS in the LPCMO system. The ability to create a pure ferromagnetic phase in manganite nanodisks is highly desirable for spintronic applications.

A large enhancement of magnetocaloric effect by chemical ordering in manganites

For conventional ferromagnetic systems, the magnetocaloric effect (MCE) is dominated by the entropy changes upon magnetic phase transitions. For strongly correlated oxides such as manganites, in which competing magnetic orders coexist and respond to an external field differently, the MCE is more complex due to the electronic phase separation (EPS) phenomenon. Taking the well-known (La2/3Pr1/3)5/8Ca3/8MnO3 (LPCMO) manganite as a model system, we investigated how the length scale of EPS phases affects the MCE. Specifically, the EPS length scale can be dramatically reduced by the spatial ordering of Pr dopants in the LPCMO system. Experimental results indicate that the magnetic entropy change of the Pr-ordered LPCMO is considerably larger than that of the Pr-random LPCMO by a factor up to six at the onset temperature of the ferromagnetic phase. A direct relation between the length scale of EPS and MCE has been established based on the experimental results.

A strain-induced new phase diagram and unusually high Curie temperature in manganites

Raising the critical temperature of functional materials is a major challenge for the exploitation of many exciting physical phenomena, such as high-Tc superconductivity, colossal magnetoresistance, and multiferroicity in strongly correlated systems. To this end, chemical doping, pressure, epitaxial strain, electric gating, interfacial charge transfer, and symmetry broken effects at the surface or edge have been used as the major means. While all these efforts have had some success, room temperature remains as the highly desirable yet difficult hurdle to clear. In this work, we demonstrate that the Curie temperature of a manganite system can be raised to over 300 K by tuning the epitaxial strain and chemical doping, and explain the underlying mechanism based on density functional theory (DFT) calculations and Monte Carlo (MC) simulations. Furthermore, we successfully designed a room temperature spin injector in a magnetic tunnel junction device based on the high-Tc manganite.